Calling an external class inside my controller

I’m trying to call a class from 3rd party code that is in my web/extensions folder, it’s at web/extensions/drip_api.php

Unfortunately, when I call the class from inside my controller, the error message says Class ‘app\controllers\Drip_Api’ not found – which tells me that it thinks it’s a controller class.

Is there anyway to use the class from the web/extensions folder without having to go through the trouble of creating an extension and so forth?

Read the guide?